That Time Of The Month
by MeanderingMotivation
Summary: Rin is adamant about teaching his little sister to swim, but Gou is being unusually stubborn.


That Time Of The Month

 **Notes:**

Hello to any readers. I hope you enjoy this poor (and slightly inappropriate) attempt at humor. For some reason I find the image of bumbling, uninformed Rin hilarious.

Have fun!

* * *

"Come on, Gou." Rin sighed, running a hand through his wet crimson hair. "I'll be right here the whole time. You won't drown. I promise."

Gou shook her head wildly. "No thank you, Onii-chan. I'm fine here."

Rin had been trying to convince his younger sister to join him in the water for some time now, and always being somewhat quick to temper, he was becoming perturbed with her continued refusal. He had been bobbing patiently in his school's pool for an hour and a half, and she still remained stubborn.

His reasoning for her learning to swim was quite simple.

Her safety.

Gou was constantly surrounded by water as the club's manager, but had never learned to brave it herself. Rin was worried that one day she might slip into the pool when no one was around and have trouble surfacing. It may have been overprotective, but she was his sister and he wanted her to be safe.

Just last week, she had come to visit him at practice (and conduct some research, the likes of which he tried to ignore), however Momotarou had gotten a little boisterous and accidentally knocked her into the water.

Luckily, Seijuro, who had been visiting, dived in to rescue her, no doubt satisfying some sort of Gou-related fantasy as he carried Rin's soaking wet sister bridal style out of the pool.

And yes, Rin did notice the appreciative glances downwards, thank you very much.

And no, he would never allow his little sister to even associate with the likes of Seijuro, who was old enough to know what he wanted, and how exactly to go about getting it.

It didn't help that Rin had noticed how Gou was gradually warming up to the male. Although she was probably only interested in his muscles…

Honestly, he didn't really know.

She was a girl, his sister, to be precise.

He didn't care about what feelings she harbored for boys as long as she didn't act on them.

She was too damn young to be gallivanting around on dates, especially with confident men like Seijuro.

Sure, everyone may have sung his praises and said that under the muscle he was _really sweet_ , but Rin didn't buy into that nonsense.

Any guy who was into his little sister was villainous in his mind.

To digress, Rin didn't want his sister drowning, nor did he want her being swept up into men's arms.

He trusted the Iwatobi boys, but everyone else was off the table. Even Momo, who he knew didn't have the slightest chance with Gou, seemed like a threat to his hypersensitive big brother eyes.

 _Bottom line: She needed to learn how to swim._

And he needed to be the one to teach her.

It was the least he could do. She'd always nagged him about it when she was younger, and he'd always waved her off, discarding her words.

Something he was deeply regretful for now. If he had managed to spare some time on her, they wouldn't be in this predicament. Her, hovering resolutely at the edge of the pool, and him, insisting tenaciously. "You have to learn." He went on mulishly. "What if you slip into the deep-end one day and there's no one around?"

"Perhaps another time?" She said, phrased more as a question. "I really don't want to swim today, Onii-san."

"Why? This is a good time. I even got a swimsuit for you." Rin said defensively, crossing his toned arms across his pectorals. It had taken him _hours_ to choose one. He knew Gou already had a swimsuit, but it was a _bikini_. A bikini! On _his_ little sister! Ridiculous. This was a _boy's_ school, for God's sake! Rin didn't want anyone wolfishly eyeing her off. She was too young to be flashing that amount of skin. Plus, it made him feel awkward. He was forced to gaze upon the evidence of her growing up.

So, he'd purchased her a modest one-piece. He hadn't known what colour to choose, so he'd selected a pink one. It had been her favorite color in their childhood years. And maybe he might have got one with flowers and hearts, but they were age appropriate!

Rin didn't miss the wince at the mention of the swimsuit. He'd presented it to her upon her arrival, and she had been less than enthusiastic about wearing it. In fact, she hadn't even tried it on yet! Utterly ungrateful. "The pool is empty asides from us. I can teach you without any interruptions."

Gou sighed. "It _really_ isn't a good time-"

"It's the perfect time! Now hurry up and get changed already. You can use the boy's dressing room since there's no one in there right now." Rin knew for sure. He'd scoped it out prior to her arrival, even checking for phones and cameras. Which, admittedly, _may_ have been bordering on paranoia. But he wouldn't put it past Momo...

Then, Rin noticed that his sister was idly rubbing her lower stomach. "Are you sick?" He questioned, scrutinising her further. Her skin wasn't flushed (although it was a little pale) and she didn't display any visible signs of nausea or pain.

Gou blinked in surprise, eyelashes fluttering. "No, Onii-san."

She wasn't lying. Gou had never been a great liar. And Rin had always been good at detecting lies. Yet, she still didn't seem completely normal…

 _Maybe she overate. She was devouring a chocolate bar when she arrived to watch practice…but it would have settled by now…_

Either way, time was running out. The pools weren't open all of the time, and Rin didn't really have time over the next week to try teaching her again. That was a lot of time in which she could inadvertently drown. "If you don't get changed in the next five minutes I will throw you in fully clothed…" Rin threatened with a scowl.

Gou balked at this, stepping back nervously and shaking her head. "I think I should go…"

"I will chase you down." Rin shot back. Indeed, he would. And he'd catch her easily. He may have been a swimmer, but he was a good sprinter. It helped that she was still wearing school shoes and a skirt. They would be hindrances.

Gou eyed him unsurely, no doubt contemplating her chances. "Onii-san-"

"If you don't have a legitimate excuse-"

"I do." Gou interrupted. "I have homework-"

"No you don't." Rin said flatly, catching the way her eyes darted away. A lie.

"-To share with a friend at a café-"

"It's getting too late for you to be out. It's a school night."

"Well maybe I should be going-"

Rin clenched his jaw, fed up. He was losing the fight with his temper. In a smooth movement, he pulled himself from the pool, dripping warm water onto the floor. He advanced forward, and rolled his eyes when Gou gave a squeak and tried to flee.

Tried, because she turned around and ran straight into the chest of Makoto, who had just entered the area alongside the other Iwatobi boys. He blinked in surprise at the contact, and reached down to steady her shoulders. "Are you okay?" He asked with concern.

Gou shook her head rapidly. "No! Onii-san is trying to throw me in the pool!"

Makoto quirked an eyebrow. "Fully clothed?"

"Sounds cool." Haruka spoke up, predicably undaunted by the prospect of swimming in clothes. As long as he could be in the water…

"What are you guys doing here?" Rin semi-growled, causing the group to exchange looks at his tone. He wanted to be pleasant, but he was just so _irritated_ with his sister.

"We came to see Rin-chan!" Nagisa chirped, beaming upwards at the taller male.

"Indeed." Rei added in agreement, pushing his glasses up the bridge of his nose.

"Why are you trying to toss Gou-kun into the pool?" Nagisa blinked innocently.

When Gou didn't offer her usual complaint about her name, he cocked his head to the side like a curious puppy. A deceptively cute puppy with a penchant for mischief. "Why are you trying to ruin away, Gou-kun? You always say how you want to learn."

"Exactly." Rin said triumphantly, grasping her arm and dragging her towards the edge. "Now get in already! It's getting late!"

Makoto opened his mouth at the rough touch, but Haruka sent him a look, and he closed it again. It was clear this was a sibling quarrel they shouldn't interrupt.

Nagisa, apparently, didn't seem to agree. His eyes were focused on Gou, lingering on her paler than usual complexion, the way she pulled at the bottom of her skirt, and the hand still pressing down on her navel. Understanding lit up his eyes, and he raced over to Rin, pushing between the two. Everyone gave a short gasp, as Rin slipped and toppled into the pool alone. He resurfaced a moment later, eyes blazing with anger. Before he could say a word (or rather, shout) Nagisa grasped Gou's hand, dragging her in the direction of the exit. "I'M BORROWING GOU-KUN!" He called behind him, fleeing the pool hurriedly.

"What the _fuck_?" Rin hissed after a moment, and Makoto frowned at the expletive. "Did he just steal my sister!?"

Haruka shrugged, and Makoto sighed. "It would seem so."

"Why the hell would he-"

"Nagisa had some kind of reason." Rei said, reminding the group of his presence. His eyes were analytical. "He was observing her closely, and something he noticed prompted him to react in that manner. I'm sure he will return soon."

Nagisa did return, sometime later. The group had been preparing to leave without him, but paused as he wandered back into the room, still smiling. Rin immediately begun cussing him out, but Makoto insisted they listen to the small swimmer's explanation.

Which, annoyingly enough, Nagisa seemed extremely vague on.

"She was having some _stomach_ pains."

"It's just that time of month, you know."

And finally, on "Her _**Aunt Flow**_ has come to visit." Rin lost his temper once more.

"I don't HAVE an Aunt Flow!" He snapped. "Nagisa, if you don't tell me the truth in the next twenty seconds, I'm going to-"

" _ **Oh."**_ Rei and Makoto voiced in unison, new dawning on their faces.

"Oh?" Haruka repeated, and Makoto hastily whispered something in his ear. Unable to express himself through eye contact this time. Haruka's eyes widened, but his face remained apathetic. "I see."

"Oh _come on_ …" Rin bemoaned, utterly fed up. "Can someone clue me in here? She's my sister!"

Like clockwork, Makoto and Haruka looked to one another.

 _He seems angry._ Haruka raised an eyebrow.

 _But it is Gou's business._ Makoto raised one in return.

 _I just wanted to swim._ Haruka gave a small sigh.

Rin gave another moan, frustrated just watching the pair. "Rei?" He snarled, and the male raised his hands defensively.

"It is not my business, Rin-san." He said stoically.

Rin then seized Nagisa by the shirt, raising him to eye-level and shaking him gruffly. "Tell me right now!" He commanded.

"Mean." Nagisa pouted at the shaking, but conceded, nevertheless. "I'm only telling you so you don't make this mistake again. Ladies can be real private."

Another shake emphasized Rin's impatience.

Nagisa took a deep breath. "Period."

"I don't care about grammar-"

This time, Nagisa rolled his eyes. "Pads."

"Shoulder pads?"

"Menstruation."

"Menstra- _what_?"

"Boy's schools…" Nagisa muttered to himself, before raising his voice. "PERIOD. MENSTRUATION. BLOOD GUSHING FROM THE VAGINA ONCE A MONTH. PADS. TAMPONS. FOOD-CRAVINGS. BREAKOUTS. MOOD-SWINGS!"

Rin, suddenly appearing rather sick, lowered Nagisa to the ground, expression horrified.

"TAMPONS ARE INSERTED-"

Rei promptly covered Nagisa's mouth. "I think Rin-san understands now." He told the smaller male. Hesitantly, he raised his hand, and Nagisa continued at a more civil level.

"She's on her period and she ran out of tampons. Girls can't go swimming with pads. That's the reason she didn't want to swim."

"How do know all of this stuff!?"

"Rin-chan, I have _sisters._ My house turns into a war zone once a month."

Rin slumped his shoulders, feeling guilty. "Why didn't she just tell me?"

"How would you have reacted?" Nagisa countered. "She felt embarrassed and awkward."

" _I_ feel embarrassed and awkward…"

Makoto patted Rin companionably on the back. "Don't worry. It'll be fine. Just respect her water-related wishes in the future."

"What if she hates me now?"

"Oh, she will." Nagisa voiced brightly, rendering Rin appalled. "But she'll only hate you once a month."

"For the rest of my life?"

"Yep. Congratulations on your sister's blooming, Rin-chan."

" _ **Fuck…"**_

"Speaking of fucking, if she skips a period, you'll know why…"

"Nagisa! Don't scare him!"

"Just being honest. I'm hungry now. Mako-chan, will you buy me ice cream? Oh, and Rin, ice cream is a good way to attempt earning Gou-kun's forgiveness."

 _I guess she really is growing up._

* * *

Sorry for the somewhat abrupt ending. Rin is finally beginning to accept his little sister is growing up. :3

(Please)Review if you can be bothered. Thanks for reading. See ya.

 **PS: I have an AO3 account, so if anyone prefers reading on there, follow the link (if it's working)**


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